1. Field of the Invention
The present invention relates to a control system for a hybrid vehicle with an engine and a motor, and in particular, to a control system for a hybrid vehicle which determines whether the output from the engine is assisted by the motor with reference to an air-fuel ratio of an air-fuel mixture which is supplied to the engine.
This application is based on Japanese Patent Application No. Hei 11-310348, the contents of which are incorporated herein by reference.
2. Description of Related Art
Conventionally, hybrid vehicles which carry electric motors as power sources for driving vehicles in addition to engines are known. One kind of conventional hybrid vehicle is a parallel hybrid vehicle which uses a motor as an assisting driving source for assisting the output from the engine. In the parallel hybrid vehicle, for example, when the vehicle accelerates the output from the engine is assisted by the motor, a battery is charged by deceleration regeneration when the vehicle decelerates, and various controls are performed while maintaining electric energy of the battery (which is hereinafter called the xe2x80x9cstate of charge (remaining battery charge)xe2x80x9d) in response to the driver""s demands (as disclosed in, for example, Japanese Unexamined Patent Application, First Publication, No. Hei 7-123509).
In an example of the above-described conventional hybrid vehicle, when an air-fuel ratio of an air-fuel mixture which is supplied to the engine is frequently shifted between the air-fuel ratio for the lean state, which is lower than the theoretical air-fuel ratio (a stoichiometric state), and the air-fuel ratio for the rich state, which is higher than the theoretical air-fuel ratio, in response to the running condition of the engine, an assist determination threshold value (a standard value which determines whether the output from the engine by the motor is assisted) is changed. Accordingly, the engine output assistance by the motor may be started or stopped.
When the air-fuel ratio of the air-fuel mixture which is supplied to the engine, is set at the lean state in which the air-fuel ratio is lower than the theoretical air-fuel ratio, that is, a lean-burn control is performed, the amount of nitrogen oxide (NOx) exhaust tends to increase. As an example of a conventional art which prevents the amount of NOx exhaust from increasing, an exhaust gas control device which includes an NOx absorbent which absorbs or adsorbs NOx and which is provided in the exhaust system of the engine to purify the exhaust gas is known.
When the air-fuel ratio of the exhaust gas is lower than the theoretical air-fuel ratio, that is, the air-fuel ratio of the exhaust gas is set at the lean state, and the oxygen concentration in the exhaust gas is relatively high, the NOx absorbent absorbs or adsorbs NOx. When the air-fuel ratio of the exhaust gas is in the vicinity of or higher than the theoretical air-fuel ratio, that is, when the air-fuel ratio is set at the rich state, and the oxygen concentration in the exhaust gas is relatively low and the concentration of hydrocarbons (HC), carbon monoxide (CO), and the like included in the exhaust gas is high, then NOx absorbed or adsorbed in the NOx absorbent is deoxidized and converted to nitrogen gas and emitted.
When the engine output assistance by the motor is repeatedly started and stopped, problems arise such that drivability decreases and drivers feel discornfort.
NOx is accumulated in the NOx absorbent, so that the purification efficiency of NOx is reduced, therefore, it is necessary to emit NOx accumulated in the NOx absorbent at a suitable time. The air-fuel ratio of the exhaust gas flowing into the NOx absorbent is temporarily set at the rich state, which is higher than the theoretical air-fuel ratio and NOx is emitted from the NOx absorbent Simultaneously, HC, CO, and the like, which are contained in the exhaust gas of the rich state, are used to control the reduction and purification of NOx emitted (hereinafter called a xe2x80x9crich spikexe2x80x9d), so that the NOx absorbent is regenerated (as is disclosed, for example, in Japanese Unexamined Patent Application, First Publication, No. Hei 10-276267).
In the control system of hybrid vehicles according to the example of the above conventional art, when the standard which determines whether to start the engine output assistance by the motor is changed depending on whether the air-fuel ratio of the air-fuel mixture which is supplied to the engine is set at the lean state, that is, the air-fuel ratio is lower than the theoretical air-fuel ratio, or at the rich state, that is, the air-fuel ratio is higher than the theoretical air-fuel ratio, the running condition of the vehicle may change whenever a rich spike control is executed.
For example, since the output from the engine is relatively low when the lean-burn control is performed, the driver may depress the accelerator pedal to maintain the drivability. If the rich spike control is executed in this condition, the engine output assistance by the motor may be suddenly started. Therefore, an unexpected acceleration or the like may be experienced by the driver.
Additionally, if such a rich spike is, for example, repeatedly executed at predetermined intervals, the vehicle""s running condition will be jerky and the vehicle will not run smoothly, and the drivability deteriorates.
It is therefore an object of the present invention to provide a control system for a hybrid vehicle which can prevent the jerky feeling in the running condition of the vehicle and can improve the drivability, even if the air-fuel ratio of the exhaust gas compared with the theoretical air-fuel ratio is temporarily changed from the lean state to the rich state, in order to regenerate the exhaust gas control device used for deoxidizing and purifying nitrogen oxide in the exhaust gas.
To achieve the above object, the control system is provided in a hybrid vehicle of the present invention with a combustion engine (E) for outputting a driving force, an electric motor (M) for generating a force for assisting the output from the engine, depending on the driving conditions, a power storage unit (battery 22) for storing electric energy generated by the motor acting as a generator using the output from the engine and electric energy regenerated by the motor when the vehicle decelerates. The control apparatus comprises: an output assist detenination means (steps S122 and S135) for determining, based on a determination threshold value (the throttle assist trigger threshold value MTHAST, the air intake passage pressure assist trigger threshold value MAST, or the air intake passage pressure assist trigger threshold value MASTTH) as the standard, whether to assist the output from the engine by the motor, depending on the driving condition of the vehicle; an air-fuel controller (FIECU 12) for changing the air-fuel ratio of the mixture, which is to be supplied to the engine, to a condition leaner or richer than the stoichiometric air-fuel ratio; a determination threshold value changing means (steps S251 and S301) for changing the determination threshold value, depending on whether the air-fuel ratio of the mixture is leaner or richer than the stoichiometric air-fuel ratio; and a determination threshold value change prohibiting means (steps S254 and S304) for prohibiting the operation of the determination threshold value changer when the air-fuel controller changes the air-fuel ratio of the mixture from a condition leaner than the stoichiometric air-fuel ratio to a condition richer than the stoichiometric air-fuel ratio.
According to the above control system for a hybrid vehicle, even in the case where the air-fuel ratio of the air-fuel mixture supplied to the engine is temporarily set richer than the stoichiometric air-fuel ratio by the air-fuel ratio controller, for example, in order to recover the absorption capacity of NOx absorbent when the absorbent is degraded, because the determination threshold value change prohibiting means forbids the operation of the determination threshold value change device, the determination threshold to be used for determining whether the electric motor assists the output of the engine remains unchanged. Therefore, the determination threshold for lean burn is continuously used in such a case, and it is possible to prevent a sudden change of driving state due to a change of the determination threshold, and the smoothness in driving vehicle can thereby be improved.
In a second aspect of the present invention, the control system for a hybrid vehicle further comprises a terminating means for terminating the prohibition of the change of the determination threshold value while the determination threshold value change prohibiting device is prohibiting the change of the determination threshold value, when the air-fuel ratio controller detects that the air-fuel ratio of the mixture is leaner than the stoichiometric air-fuel ratio, or when the prohibition of the change of the determination threshold value is maintained for a specified time.
According to the second aspect of the control system for a hybrid vehicle, after the rich-spiking for temporarily setting the air-fuel ratio of the air-fuel mixture richer than the stoichiometric air-fuel ratio is performed, when the air-fuel ratio is reset to a value leaner than the stoichiometric air-fuel ratio, or when a predetermined period of time has passed from the beginning of the prohibition of the threshold value change, the terminating device determines that the air-fuel ratio control by the rich-spiking is complete, and the terminating device terminates the prohibition of the threshold value change. Therefore, it is possible to automatically terminate the prohibition of the threshold value change.
In a third aspect of the present invention, the control system is provided in a hybrid vehicle with a combustion engine (E) for outputting a driving force, an electric motor (M) for generating a force for assisting the output from the engine, depending on the driving conditions, a power storage unit (battery 22) for storing electric energy generated by the motor acting as a generator using the output from the engine and electric energy regenerated by the motor when the vehicle decelerates. The control apparatus comprises: an output assist determination means (steps S122 and S135) for determining, based on a determination threshold value (the throttle assist trigger threshold value MTHAST, the air intake passage pressure assist trigger threshold value MAST, or the air intake passage pressure assist trigger threshold value MASTTH) as the standard, whether to assist the output from the engine by the motor, depending on the driving condition of the vehicle; an air-fuel controller (FIECU 12) for changing the air-fuel ratio of the mixture, which is to be supplied to the engine, to a condition leaner or richer than the stoichiometric air-fuel ratio; a determination threshold value changing means (steps S251 and S301) for changing the determination threshold value, depending on whether the air-fuel ratio of the mixture is leaner or richer than the stoichiometric air-fuel ratio; an exhaust cleaner (40) having an oxygen concentration measurement device (S9), provided in an exhaust system of the engine, for measuring the oxygen concentration in the exhaust gas, and a nitrogen oxide reduction device (NOx absorption capacity 42) for absorbing nitrogen oxide in the exhaust gas when the oxygen concentration in the exhaust gas is high and for reducing the absorbed nitrogen oxide when the oxygen concentration in the exhaust gas is low; a reduction means (FIECU 12 also performs this function) for setting the air-fuel ratio of the mixture to a condition richer than the stoichiometric air-fuel ratio so as to reduce the oxygen concentration in the exhaust gas, when the air-fuel ratio of the mixture, which is to be supplied to the engine, is leaner than the stoichiometric air-fuel ratio; and a determination threshold value change prohibiting means (steps S254 and S304) for prohibiting the operation of the determination threshold value changing means when the reduction device changes the air-fuel ratio of the mixture from a condition leaner than the stoichiometric air-fuel ratio to a condition richer than the stoichiometric air-fuel ratio.
According to the third aspect of the control system for a hybrid vehicle, it can be determined whether the air-fuel ratio of the exhaust gas flowing into the nitrogen oxide reduction device is leaner than the stoichiometric air-fuel ratio, and based on the result of this determination, the air-fuel ratio of the mixture gas to be supplied to the engine is controlled. Therefore, it is possible to precisely detect the state of the air-fuel control by a rich-spiking, and the degraded nitrogen oxide reduction device can thereby be securely regenerated. Therefore, the driving state of the vehicle can be precisely determined, the change or prohibition of change of the determination threshold can be controlled based on a precise determination, thus, the smoothness in driving the vehicle can be improved.
In a fourth aspect of the present invention, the control system for a hybrid vehicle according to claim 3 further comprises: a vehicle speed measuring device (vehicle speed sensor S1) for detecting the speed of the vehicle. The reduction means sets the air-fuel ratio of the mixture to the condition richer than the stoichiometric air-fuel ratio so as to reduce the oxygen concentration in the exhaust gas at a time interval depending on the vehicle speed detected by the vehicle speed measuring device.
According to the fourth aspect of the present invention, because the degree of the deterioration of the nitrogen oxide reduction means is indirectly determined based on the driving state of the vehicle, and rich-spiking is performed at a time interval depending on the vehicle speed detected by the vehicle speed measuring device, it is possible to set an appropriate frequency of the rich-spiking and to improve the smoothness in driving the vehicle.
In a fifth aspect of the present invention the control system for a hybrid vehicle further comprises a terminating means (steps S256 and S306) for terminating the prohibition of the change of the determination threshold value while the determination threshold value change prohibiting means prohibits the change of the determination threshold value, when the air-fuel ratio controller detects that the air-fuel ratio of the mixture is leaner than the stoichiometric air-fuel ratio, or when the prohibition of the change of the determination threshold value is maintained for a specified time.
According to the fifth aspect of the present invention, after the rich-spiking for setting the air-fuel ratio of the air-fuel mixture richer than the stoichiometric air-fuel ratio is performed so as to relatively decrease the oxygen concentration in the exhaust gas, when the air-fuel ratio is reset to a value leaner than the stoichiometric air-fuel ratio, or when a predetermined period of time has passed from the beginning of the prohibition of the threshold value change, the terminating means determines that the air-fuel ratio control by the rich-spiking has been completed, and the terminating means terminates the prohibition of the threshold value change. Therefore, it is possible to automatically terminate the prohibition of the threshold value change.